Illustration of a geothermal heating and cooling system that handles multiple loads for a community. Illustration by Sarah Cheney.

Illustration of a geothermal heating and cooling system that handles multiple loads for a community. Illustration by Sarah Cheney.

Imagine a home in which the temperature is always comfortable, yet the heating and cooling system is out of sight. That system performs efficiently but doesn’t require extensive maintenance or knowledge on the part of the owners.

The air smells fresh; you can hear the birds chirping and the wind rustling lazily through the trees. The home shares energy with the earth similar to the way the roots of the trees exchange the essentials of life to their leaves and branches. Sounds comfortable, doesn’t it?

Geothermal heating and cooling makes that vision a reality. Geothermal HVAC (heating, ventilating, and air conditioning) brings a building in harmony with the earth beneath, taking advantage of subterranean temperatures to provide heating in the winter and cooling in the summer.

How Geothermal Heating and Cooling Works

Outdoor temperatures fluctuate with the changing seasons but underground temperatures don’t change as dramatically, thanks to the insulating properties of the earth. Four to six feet below ground, temperatures remain relatively constant year-round. A geothermal system, which typically consists of an indoor handling unit and a buried system of pipes, called an earth loop, and/or a pump to reinjection well, capitalizes on these constant temperatures to provide “free” energy.

(Note that geothermal HVAC should not be confused with “geothermal energy,” the process by which electricity is generated directly from the heat inside the earth. That takes place on the scale of utilities and uses different processes, normally by heating water to boiling.)

The pipes that make up an earth loop are usually made of polyethylene and can be buried under the ground horizontally or vertically, depending on the characteristics of the site. If an aquifer is available, engineers may prefer to design an “open loop” system, in which a well is drilled into the underground water. Water is pumped up, run past a heat exchanger, and then the water is returned to the same aquifer, through “reinjection.”

Diagram of how geothermal HVAC systems work

Diagram of how geothermal HVAC systems work. Illustration from Modern Geothermal HVAC

In winter, fluid circulating through the system’s earth loop or well absorbs stored heat from the ground and carries it indoors. The indoor unit compresses the heat to a higher temperature and distributes it throughout the building, as if it were an air conditioner running in reverse. In summer, the geothermal HVAC system pulls heat from the building and carries it through the earth loop/pump to reinjection well, where it deposits the heat into the cooler earth/aquifer.

Unlike ordinary heating and cooling systems, geothermal HVAC systems do not burn fossil fuel to generate heat; they simply transfer heat to and from the earth. Typically, electric power is used only to operate the unit’s fan, compressor, and pump.

A geothermal cooling and heating system has three main components: the heat-pump unit, the liquid heat-exchange medium (open or closed loop), and the air-delivery system (ductwork) and/or the radiant heating (in the floor or elsewhere).

Geothermal heat pumps, as well as all other types of heat pumps, have efficiencies rated according to their coefficient of performance, or COP. It’s a scientific way of determining how much energy the system moves versus how much it uses. Most geothermal heat pump systems have COPs of 3.0 to 5.0. This means for every unit of energy used to power the system, three to five units are supplied as heat.

Geothermal systems require little maintenance. When installed properly, which is critical, the buried loop can last for generations. The unit’s fan, compressor, and pump are housed indoors, protected from the harsh weather conditions, so they tend to last for many years, often decades. Usually, periodic checks and filter changes and annual coil cleaning are the only required maintenance.

Geothermal HVAC Spreads

Geothermal HVAC systems have been used for more than 60 years in the U.S. and beyond.

They work with nature, not against it, and they emit no greenhouse gases. (As mentioned earlier, they use a smaller amount of electricity to run, because they are coupled in with the earth’s average temperature.)

Geothermal HVAC systems are becoming common features of eco-friendly homes as part of the growing green building movement. Green projects accounted for 20 percent of all newly built homes in the U.S. last year. By 2016, a Wall Street Journal article predicted that green housing will grow from $36 billion a year to as much as $114 billion. That’s approaching 30 to 40 percent of the entire housing market.

But a lot of information out there on geothermal heating and cooling is based on outdated information, or outright myths. In our new book Modern Geothermal HVAC Engineering and Control Applications (Egg/Cunniff/Orio -McGraw-Hill 2013), co-authors Greg Cunniff, Carl Orio and I bust many of these myths.

Geothermal HVAC Myths Busted

1.     Geothermal HVAC systems are not considered a renewable technology because they use electricity.

Fact: Geothermal HVAC systems use only one unit of electricity to move up to five units of cooling or heating from the earth to a building.

2.     Photovoltaic and wind power are more favorable renewable technologies when compared to geothermal HVAC systems.

Fact: Geothermal HVAC systems remove four times more kilowatt-hours of consumption from the electrical grid per dollar spent than photovoltaic and wind power add to the electrical grid. Those other technologies can certainly play an important role, but geothermal HVAC is often the most cost effective way to reduce environmental impact of conditioning spaces.

3.     Geothermal HVAC needs lots of yard or real estate in which to place the polyethylene piping earth loops.

Fact: Depending on the characteristics of the site, the earth loop may be buried vertically, meaning little above-ground surface is needed. Or, if there is an available aquifer that can be tapped into, only a few square feet of real estate are needed. Remember, the water is returned to the aquifer whence it came after passing over a heat exchanger, so it is not “used” or otherwise negatively impacted.

4.     Geothermal HVAC heat pumps are noisy.

Fact: The systems run very quiet and there is no equipment outside to bother neighbors.

A technician inspects a geothermal HVAC air handler

A technician inspects a geothermal HVAC air handler. Photo courtesy of Jay Egg

5.     Geothermal systems eventually “wear out.”

Fact: Earth loops can last for generations. The heat-exchange equipment typically lasts decades, since it is protected indoors. When it does need to be replaced, the expense is much less than putting in an entire new geothermal system, since the loop or well is the most pricey to install. New technical guidelines eliminate the issue of thermal retention in the ground, so heat can be exchanged with it indefinitely. In the past, some improperly sized systems did overheat or overcool the ground over time, to the point that the system no longer had enough of a temperature gradient to function.

6.     Geothermal HVAC systems only work in heating mode.

Fact: They work just as effectively in cooling and can be engineered to require no additional backup heat source if desired, although some customers decide that it is more cost effective to have a small backup system for just the coldest days if it means their loop can be smaller.

7.     Geothermal HVAC systems cannot heat water, a pool, and a home at the same time. Fact: Systems can be designed to handle multiple loads simultaneously.

8.     Geothermal HVAC systems put refrigerant lines into the ground.

Fact: Most systems use only water in the loops or lines.

9.     Geothermal HVAC systems use lots of water.

Fact: Geothermal systems actually consume no water. If an aquifer is used to exchange heat with the earth, all the water is returned to that same aquifer. In the past, there were some “pump and dump” operations that wasted the water after passing over the heat exchanger, but those are exceedingly rare now. When applied commercially, geothermal HVAC systems actually eliminate millions of gallons of water that would otherwise have been evaporated in cooling towers in traditional systems.

10.  Geothermal HVAC technology is not financially feasible without federal and local tax incentives.

Fact: Federal and local incentives typically amount to between 30 and 60 percent of total geothermal system cost, which can often make the initial price of a system competitive with conventional equipment. Standard air-source HVAC systems cost around $3,000 per ton of heating or cooling capacity, during new construction (homes usually use between one and five tons). Geothermal HVAC systems start at about $5,000 per ton, and can go as high as $8,000 or $9,000 per ton. However, new installation practices are reducing costs, to the point where the price is getting closer to conventional systems under the right conditions.

Factors that help reduce cost include economies of scale for community, commercial, or even large residential applications and increasing competition for geothermal equipment (especially from major brands like Bosch, Carrier, and Trane). Open loops, using a pump and reinjection well, are cheaper to install than closed loops.

Thanks for the thousands of likes and hundreds of comments!  National Geographic has closed comments for this blog.  Please continue the conversation and get the answers you need for geothermal heating and cooling on Jay Egg’s blog, “Geothermal Heating and cooling Questions and Answers

Jay Egg is the co-author of the new book Modern Geothermal HVAC Engineering and Control Applications (McGraw-Hill 2013), with Greg Cunniff and Carl Orio. He co-wrote the book Geothermal HVAC, Green Heating and Cooling in 2010 with National Geographic’s Brian Clark Howard. Jay consults with the geothermal HVAC industry. He previously served as an installer of the technology through his company EggGeothermal.

Comments

  1. Mike S
    Florida and BAhamas
    December 4, 2013, 1:09 am

    Can I put a system in using a salt water well in the Bahamas? I LOVE the systems when the water source is realiable. In a closed looop system, how much water needs to be in the Cisterns or the Loop? Can you advise or consult?
    Thanks,

  2. Jay Egg
    Florida
    December 3, 2013, 4:50 pm

    Miles,
    First of all, geothermal is worth doing in your home whether you go with radiant or forced air heating. Your question has to do with whether a staple up radiant system performs well enough to consider radiant heating. By most accounts, there is a greater lag time when using staple up Pex tubing technology, simply because conduction of the heat to the floor can take longer. Still, fundamentally all of the BTU’s are reaching your living space, unless they are escaping to an unconditioned basement or other space below. That has more to do with installation practices underneath the staple up hydronic tubng, if adjacent to an unconditioned space. Generally, occupants like to feel a floor that is warm to the touch – and with staple up, the floor may have a smaller difference in temperature when compared to the conditioned space because of the more gradual heat transfer.
    Please feel free to email for further clarification. jayegg.geo@gmail.com
    Jay Egg

  3. Miles Kunkel
    Boulder CO
    December 2, 2013, 4:17 pm

    -We are building a new home that has been designed and engineered with staple-up radiant heat. We have talked with a few local Geo contractors and a few have advised that in order for Geo/heat pump to work, we should only use light gypsum for the radiant system. I can understand their point as a lower water temp is used with this type of system, however we have very restrictive building codes in BLDr that affect your ceiling height that we are trying to max at 11′.
    So, if one uses staple-up, is it even worth doing Geo?Any input/advise? We will have a triangle boiler as a back-up/supplement. Thanks!

  4. Jay Egg
    Florida
    December 2, 2013, 4:03 pm

    Vinod,

    I have worked with and written about agricultural application of geothermal HVAC technologies many times in the past. This is a suitable application, and provides significant energy reduction as compared to standard cooling practices. We can attain to 60% relative humidity at 80°F using proper controls and design parameters, while reducing kWh consumption.

    The system should be designed using fluid/hydronic heat transfer. This can be accomplished through convective coils to meet your needs here.

    Jay Egg jayegg.geo@gmail.com

  5. vinod
    india
    November 30, 2013, 11:16 am

    I forgot to tell u that I also need relative humidity 60% throught the year. eagry awaiting for your responce sir.
    thank you

  6. vinod
    india
    November 30, 2013, 10:50 am

    sir im working with poultry industry both hatchery and farms need fresh air at about 80 Fahrenheits. I want to install geothermal air conditioning to get that temperature specialy using air as refrigerat as we need abundance of air per minute as high as 500 feet per minutes. and even more in farms. so i dont think water based system csn solve my problems. I want to know your openion how csn I attain such geothermal HVAC system using air and pipes should be used which will be good therman enery exchangers, what should be its diameter and how thick it should be. and what lenght should it have to get the temperature down from 120 Fahrenheit to about 80 farhenight. I actually want to know the entire process if air can be used to cool down my inlet areas down to that level. it will be a great help if u could help me with this problem. thank you sir

  7. Jay Egg
    Florida
    November 18, 2013, 10:36 am

    William Poole- I would refer you back to the answer I gave to Becky (MO) on page 4 of these comments. The only exception of which I know is that if your geothermal heating system is sized too closely to your load and does not recover quickly enough for your liking. Properly sized, you will be able to afford normal setbacks on temperature to save energy. Please let me know if this answers your query.

    Thank you-Jay Egg- jayegg.geo@gmail.com

  8. Jay Egg
    Florida
    November 18, 2013, 10:29 am

    James Guirdy-Normally on large commercial installations, and those that are cooling dominant, we use “class V thermal thermal exchange wells”. I have consulted on projects up to 6000 tons in hot and humid tropical climates that function remarkably well. Please view this power point for engineers: http://www.fefpa.org/pdf/Summer2012/PC%20Geothermal_FEFPA%20power%20point%20070212.pdf , and this news article that shows a remediation of just such an issue as you’ve depicted, through the implementation “class V” technology: http://www.negpa.org/wp-content/uploads/2013/11/Sussex-EOC-EggGeo-Complete-News-Story-2-20131004.pdf . Please note that I favor whatever technology is suited for the particular application, whether closed loop, pump to reinjection, or standing column wells. The book, “Modern Geothermal HVAC Engineering…”, just released from McGraw-Hill Professional ( http://www.amazon.com/Modern-Geothermal-Engineering-Control-Applications/dp/0071792686/ref=tmm_hrd_title_0 ) does a good job of explaining the variations for consideration.

    Thank you,

    Jay Egg

  9. William Poole
    Farmington, CT
    November 17, 2013, 4:32 pm

    We had a geothermal system installed in 2012. We have a larger house and high ceilings — thus, I previously had been in the habit of dropping the heat significantly at night to save energy costs. However, I had been told to find a comfortable temperature for the winter — and just keep it stable. I was told that in comparison to fossil fuel systems (in which the aforementioned procedure makes sense) that keeping a geothermal system at a stable temperature will save money in comparison to dropping the temperature at night. This seems quite counterintuitive to me. Can you explain if this is correct ? If so, what is the science behind it ? Thank you !

  10. Jamie Guidry, P.E.
    Arkansas
    November 16, 2013, 4:32 pm

    @ Mr. McInnis – You might consider a variation of the mini-split product called a variable refrigerant (VRF) systems. VRF is fairly new to North America, but has been successfully installed in Europe and Asia since the 1970’s. Daikin, Mitsubishi, and LG all have great products and will give you competitive pricing. VRF heat pump systems employ a variable-speed compressor technology that will allow you to have adequate heating down to -10 degrees F, which means they don’t require auxiliary electric heat. They also have the ability to submeter individual tenants as a factory controls option. I recommend you ask around your local market about the three manufacturer’s mentioned above. Good Luck.

    Jamie Guidry, P.E.
    National Engineering Services, PLLC

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